The characteristics of a CMOS (complementary metal-oxide-semiconductor) image sensor or CCD (charge coupled device) image sensor or other image input sensors have been improved, and the demand for them in the fields of digital cameras and camera-attached cellular phones, etc. has increased. 1.
It is desired to further improve the characteristics of said image sensors. For example, it is desired to expand their dynamic range.
Solid-state image pickup devices designed to expand the dynamic range are disclosed in Japanese Kokai Patent Application No. 2003-134396, Japanese Kokai Patent Application No. 2000-165754, Japanese Kokai Patent Application No. 2002-77737 and Japanese Kokai Patent Application No. Hei 5[1993]-90556. However, it is difficult for these solid-state image pickup devices to have an expanded dynamic range while keeping high sensitivity and a high S/N ratio. The solid-state image pickup device described in Japanese Kokai Patent Application No. 2005-328493 was developed in order to solve this problem.
In the solid-state image pickup device described in Japanese Kokai Patent Application No. 2005-328493, the photocharge overflowing from the photodiode of each pixel is stored in a floating diffusion and an electrostatic capacitance element. Signal S1 is read from the photoelectrons in the photodiode when photoelectrons are not overflowing from the photodiode. When overflow occurs, the photoelectrons in the photodiode are combined with the photoelectrons overflowing from the photodiode to read signal S1+S2 as the signal of each pixel.
In the solid-state image pickup device described in Japanese Kokai Patent Application No. 2005-328493, the floating diffusion in each pixel is connected to the gate electrode of an amplification transistor known as a source follower. The photocharge is converted into a voltage and is subjected to current amplification as the current between the source and drain of the transistor in the source follower. The signal level is maintained in the analog memory part in the solid-state image pickup device and is read out for each line.
In this case, the gain of the source follower of a pixel is not 1 due to a substrate bias effect. If the gain of the source follower is represented by A, a gate capacitance expressed as (1−A)*(Cox+Cgs) is added to the parasitic capacitance of the floating diffusion.
Also, since the threshold value of the source follower varies depending on the signal level due to the substrate bias effect, the output of the pixel is nonlinear. In order to obtain gain of signal S1 in the pixel part, the gain of the source follower must be close to 1. In order to realize this objective, a method that connects the source potential to a well in the transistor to reduce the substrate bias effect is well known. In fact, however, it is difficult to form a well that applies a source voltage individually in a pixel.
If the capacitance of the floating diffusion is reduced excessively, the capacitance of the floating diffusion becomes smaller than that of the photodiode. As a result, the saturation level of signal S1 drops.
The problem to solve is that it is difficult to improve the linearity of signal S1 and increase the saturation level in a solid-state image pickup device with an expanded dynamic range.